The first kind is more common: You still pay more total for the creatures if you play them face down and then flip them, but since they usually cost six to turn face up and you often wont be at six mana on turn six, you can probably still get these guys on the table easier if you use their morph ability
Do I get to count Now I Know My ABC's when counting letters for the win condition
What are the weaknesses and retreat cost of this Krookodile card
By sacrificing your own board with it on the stack, and by pre-emptying your opponents yards with anything from a Soul-Guide Lantern to an Author of Shadows, you can simultaneously wipe the board, get a load of EtBs, and slingshot into the lead
Who knows whats on the cards here
Goblin Spy: This Goblin Spy has shadow and can't be blocked, symbolizing its sneaky espionage abilities